Preparation of Alkanes | Hydrocarbons Class 11

Preparation of Alkanes

Methods, Reactions & Limitations | Hydrocarbons Class 11

1. From Unsaturated Hydrocarbons (Hydrogenation)

Alkenes and Alkynes react with Dihydrogen gas ($H_2$) in the presence of finely divided catalysts like Platinum (Pt), Palladium (Pd), or Nickel (Ni) to form Alkanes.

                    Reaction (Sabatier-Senderens):

                    $$ CH_2=CH_2 + H_2 \xrightarrow{Ni/Pt/Pd} CH_3-CH_3 $$

Note: Pt and Pd catalyze the reaction at room temperature, while Ni requires high temperature and pressure.

2. From Alkyl Halides

A. Wurtz Reaction

Alkyl halides react with metallic sodium in dry ether to form symmetrical alkanes containing double the number of carbon atoms.

$$ 2R-X + 2Na \xrightarrow{\text{Dry Ether}} R-R + 2NaX $$

Example: $2CH_3Br + 2Na \rightarrow CH_3-CH_3 + 2NaBr$

                    Limitation: Not suitable for preparing unsymmetrical alkanes (odd number of carbons) because a mixture of alkanes is formed ($R-R, R'-R', R-R'$), which is difficult to separate. Methane ($CH_4$) cannot be prepared.
                

B. Reduction with Zn/HCl

Alkyl halides on reduction with Zinc and dilute Hydrochloric acid give alkanes.

$$ R-X + H_2 \xrightarrow{Zn, H^+} R-H + HX $$

3. From Carboxylic Acids

A. Decarboxylation (Laboratory Method)

Sodium salts of carboxylic acids are heated with Soda-lime (Mixture of $NaOH$ and $CaO$ in 3:1 ratio).

$$ R-COONa + NaOH \xrightarrow{CaO, \Delta} R-H + Na_2CO_3 $$

Key Point: The alkane formed contains one carbon less than the carboxylic acid (Step-down reaction).

B. Kolbe's Electrolytic Method

Electrolysis of aqueous solution of sodium or potassium salt of carboxylic acid gives alkanes containing an even number of carbon atoms at the Anode.

$$ 2R-COOK + 2H_2O \xrightarrow{\text{Electrolysis}} R-R + 2CO_2 + H_2 + 2KOH $$

At Anode: $R-R$ (Alkane) and $CO_2$. (Free Radical Mechanism).
At Cathode: $H_2$ gas.
Solution: Becomes alkaline due to $KOH$.
Note: Methane cannot be prepared by this method.

4. From Grignard Reagent

Grignard reagents ($RMgX$) are organometallic compounds. They are extremely reactive towards any source of proton (active hydrogen) like water, alcohol, or amines to give alkanes.

$$ R-Mg-X + H-OH \rightarrow R-H + Mg(OH)X $$

Example: $CH_3MgBr + H_2O \rightarrow CH_4 + Mg(OH)Br$.

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